Supplemental lipids are commonly included in diets for ruminants (Doreau and Ferlay 1994). These lipids are components of feedstuffs or through the addition of specific lipid additives (Loften et al. 2014). Most commonly, these lipid additives are used to increase the energy density of the diet (Hess et al. 2008), but can also be used to modulate the fatty acid (FA) composition of body tissues or induce metabolic changes. Verdugo (2016) reported changes in rumen tissue FA composition as well as short-chain fatty acid (SCFA) absorption when supplementing Holstein steers fed diets with differing supplements (canola vs. flax oil) and for diets with differing lipid content. In that study, steers supplemented with more unsaturated FA (flax) tended to have higher proportion of unsaturated FA (C18:2) in rumen tissue but had lesser rates of propionate uptake via passive diffusion and total uptake of butyrate. Additionally, steers supplemented with saturated FA tended to have greater butyrate uptake via passive diffusion. While that study showed that FA concentration and saturation can affect function of the gastrointestinal tract, it is not clear which FA may induce an effect. The hypothesis is that ruminal tissue FA composition will mirror that of dietary supply, and dietary unsaturated FA supplements high in conjugated linoleic acid will increase permeability and decrease SCFA absorption as compared to FA supplements high in unsaturated C18:0. The objective is to characterize the effects that FA supplements of differing C18 FA isomers will have on ruminal tissue FA composition, total tract barrier function and absorption of SCFA across the reticulo-rumen.
Materials and Methods
Eight cannulated Hereford crossbred heifers were blocked by weight and organized in a duplicate 4 × 4 Latin square design. The sequence of treatments was balanced for carry-over effects. Heifers were assigned to one of four treatments: CON (no supplemental FA), SAT (supplemental stearic, 70% C18:0; Industrene 7018, PMC Biogenix, Memphis TN, USA), UNSAT (supplemental linoleic, 74% C18:2, 38% CLA; Pamolyn 300, Eastman, Kingsport TN, USA), or CON-ISO (no supplemental FA with diets formulated to be isocaloric to both FA diets). Experimental periods consisted of 28 d in duration. Day 1 to 7 was a washout period with diets formulated to be slightly deficient in energy (NEg = 0.73 Mcal/kg) to promote lipid reserve mobilization and limit carryover effects from prior periods. From d 8 to 28, heifers were fed their respective treatment diets (Table 1) at a fixed rate (2.25% BW on DM basis). Experimental diets were adjusted to have a DM of 75% using water to reduce the risk for feed sorting.
Heifers were supplemented with FA from d 8 to 28 at a fixed rate of 0.06% body weight, equating to 2.73% of dietary DMI. SAT heifers received their dose directly into the rumen while UNSAT heifers were abomasally infused to avoid biohydrogenation. CON and CON-ISO heifers received a sham treatment by briefly locating the reticulo-omasal orifice. Heifers were infused three times daily (0900, 1200, and 1500 h) with each dose being 33.3% of total daily allocation.
Table 1. Ingredient and predicted chemical composition of diets for heifers supplemented with no fatty acid (CON), 2% DMI of C18:0 (SAT), 2% DMI of C18:2 (UNSAT), or no fatty acid but isocaloric to lipid treatments (CONISO).
On d 8 and 28 of each period, rumen tissue biopsies were collected at 0700 h according to Steele et al. (2009). Venous blood samples were collected at 0800 h. Rumen tissue samples and plasma will be analyzed for FA composition by gas chromatography (Surkhija and Palmquist 1988). Blood non-esterified fatty acids, beta-hydroxybutyrate, and cholesterol will be measured using commercial kits. Total tract permeability will be evaluated according to Zhang et al. (2013) from d 25 to 27. Urine Cr concentration will be measured as described by (Vicente et al. 2004). On d 28, absorption of SCFA was measured by the temporarily isolated and washed reticulo-rumen method (Care et al. 1984) with buffers described by Zhang et al. (2013). Buffer was sampled at 0, 5, and 45 min. Samples will be analyzed for Cr (Vicente et al. 2004) and SCFA (Khorasani et al. 1996) concentrations to enable calculation of SCFA disappearance.
Data will be analyzed as a Latin square design using the mixed model of SAS (version 9.4, Cary, NC, USA). Treatment, square, and period will be considered as fixed effects with the randomized effect of heifer. Means will be separated using Tukeys means separation test.